1. Field of the Invention
The present invention relates to technology for manufacturing a semiconductor device, more particularly, to an exposure system, an exposure method and a method for manufacturing a semiconductor device.
2. Description of the Related Art
In a photolithography process for manufacturing a semiconductor device, illumination light emitted from an effective illumination source (secondary illumination source) formed in an illumination optical system illuminates a mask pattern provided on a photomask (reticle). The illumination light diffracted at the photomask is concentrated on a resist film applied on a semiconductor substrate by a projection optical system. An optical image is formed on the resist film, and the resist film is exposed by the optical image. The resist film is then developed, thus forming a resist pattern.
A light intensity distribution of the light illuminating the photomask is represented by a light intensity (luminance) distribution (hereinafter, referred to as illumination) of the effective illumination source. The illumination can be controlled by a zoom lens or a diffractive optical element on an incident side of a fly-eye lens or a filter (illumination aperture) provided on an effective illumination source plane. The shape of the illumination that is suitable for the shape of the resist pattern, such as normal illumination or annular illumination, is formed by controlling an illumination coherence factor σ indicating size of the effective illumination source.
In the photolithography process, for example, in the case of manufacturing a flash memory, a row of hole patterns cyclically aligned in one direction is formed as the resist pattern. One cycle of the repeating hole patterns in the alignment direction (hereinafter, referred to as a cyclic direction) has a length not more than the wavelength of light on the substrate, which is close to a resolution limit of the exposure system. Such a fine resist pattern is difficult to form even using the normal or annular illumination.
To form a fine resist pattern, a “tripole illumination” including three regions of maximum light intensity is used as the effective illumination source. The tripole illumination includes a center region of maximum light intensity located in the center of the effective illumination source and two eccentric regions of the maximum light intensity located near the edge of the effective illumination source, so as to be opposite to each other across the center region. For example, in forming the row of hole patterns, the direction that the center region and the two eccentric regions are aligned is set parallel to the cyclic direction of the rows of hole patterns. This makes it possible to obtain a resolution effective for forming the row of fine hole patterns and reduce dimensional variation (dimensional errors) of the hole patterns due to exposure amount errors and focus errors. In other words, exposure with a large process window can be achieved.
However, the patterns are increasingly miniaturized, and in the case of forming a fine resist pattern such as a row of hole patterns having a cycle length not more than three quarter wavelength, a sufficient process window cannot be obtained in some cases, even using the tripole illumination. Thus, there is a problem of increasing dimensional variation in the resist pattern.